The present invention relates generally to optical image bar printing, and more particularly to an electrode array configuration for an optical image bar.
As a matter of definition, an "optical image bar" comprises an array of optical pixel generators for converting a spatial pattern, which usually is represented by the information content of electrical input signals, into a corresponding optical intensity profile. Although there are a variety of applications for such devices and a number of different fields, a significant portion of the effort and expense that have been devoted to their development has been directed toward their application to electrophotographic printing.
One type of image bar is based on the use of electrooptic (EO) total internal reflection (TIR) spatial light modulators, as described in U.S. Pat. No. 4,396,252 to W. D. Turner, hereby incorporated by reference. The modulator comprises a set of laterally separated, individually addressable electrodes, which are maintained closely adjacent a reflective surface of an optically transparent EO element, such as a lithium niobate crystal. In operation, substantially the full width of the EO element is illuminated by a transversely collimated light beam. This light beam is applied to the EO element at a near grazing angle of incidence with respect to its reflective surface, and is brought to a wedge-shaped focus on that surface so that it is totally internally reflected therefrom.
Voltages representing a linear pixel pattern are applied to the individually addressable electrodes, whereby localized fringe electric fields are coupled into the EO element. These fields produce localized variations in the refractive index of the EO element, so the wavefront of the light beam is spatially phase modulated in accordance with the pixel pattern as it passes through the EO element. The process is repeated for a sequence of pixel patterns, with the result that the wavefront of the light beam is spatially modulated as a function of time in accordance with successive ones of those patterns.
For image bar applications of such a modulator, schlieren optics are employed to convert the phase modulated wavefront of the light beam into a corresponding series of optical intensity profiles. If a printing function is being performed, these intensity profiles are in turn used to expose a photosensitive recording medium, such as a xerographic photo receptor, in accordance with the image defined by the successive pixel patterns.
U.S. Pat. No. 4,940,314, issued Jul. 10, 1990, to D. L. Hecht, hereby incorporated by reference, addresses the problem that the effective diameter of the pixels produced by an EO image bar, as measured between their half power points at unity magnification, is approximately one-half the center-to-center spacing of its electrodes. Accordingly, such image bars not only tend to cause image distortion because of spatial quantitization errors, but also characteristically produce inter pixel intensity nulls.
To the extent possible, it is desired that there be an ON pixel for each voltage step between adjacent electrodes, and that individual pixels be of substantially the same shape and size regardless of the data pattern. Moreover, an EO image bar should be characterized by a high level of light throughput and a modest level of required drive voltage. Size is an issue as well, with desired compactness militating toward short electrodes but ease of alignment militating toward long electrodes. As is usually the case, it is difficult to achieve all these objectives simultaneously. Attempts to optimize a given characteristic tend to lead to increased interpixel crosstalk or a degradation in some other performance characteristic.